1. Field of the Invention
The present invention relates generally to methods of operating a photodetector system used to obtain radiographic information from modulation of a uniform X-ray flux by a test object or patient. More specifically, the present invention relates to methods impress a surface charge on a photon detector and produce a latent image in a photoconductor.
2. Background of the Prior Art
a. Facsimile and Vidicons
Facsimile systems modulate an electrical signal in response to light that is reflected from a small portion of an image. Facsimile requires production of a real image.
Vidicon tubes store a latent image as a charge distribution pattern in a semiconductor target. This latent image is scanned by an electron beam and the current variation induced by the different charges on separate portions of the target comprises a video signal. Vidicons require use of a high vacuum and precise focusing of an electron beam that must be shielded from external electric and magnetic fields.
b. Conventional Radiographic Methods
Latent images stored in silver halide film or selenium xeroradiographic plates must be chemically or powder cloud developed to produce real images. Use of calcium tungstate crystals or a high atomic weight gas for image intensification degrades image quality and still requires exposures of from 1 to 5 R per typical clinical mammogram. Powder cloud development of latent xeroradiographic images requires high differential charge densities on the xeroradiographic plate to attract and hold powder particles prior to fusing. This high charge differential is generated by x-rays, or ions, impinging on the surface of a charged selenium plate. The higher the differential charge needed to produce an image, the more X-ray exposure is required.
The reader is directed to the following publications for detailed discussion of this prior art.
XERORADIOGRAPHY, J. W. Boag, Phys. Med. Biol., 1973, Vol. 18, No. 1, pp. 3-37; Principals of Radiographic Exposure and Processing, Arthur W. Fuchs, 1958, Chapter 14, "X-Ray Intensifying Screens", pp. 158-164.
U.S. Pat. No. 3,860,817; Reducing Patient X-Ray Dose During Fluoroscopy With an Image System.
U.S. Pat. No. 3,828,191, Gas Handling System for Electronradiography Imaging Chamber.
U.S. Pat. No. 3,308,233, Xerographic Facsimile Printer Having Light Beam Scanning and Electrical Charging With Transparent Conductive Belt.
Recent research by the present inventors and others indicates that a detailed latent image can be created in a xeroradiographic plate by very low levels (under 10 mR) of X-ray exposure. At this exposure level latent image is present but, its associated electrostatic field is not of sufficient intensity to produce a real image by the powder cloud method of image development. For details of this research see, Grant Application, "Radiomammography With Less than 150 mR Per Exposure", available from the Department of Experimental Radiology, M.D. Anderson Hospital, Houston, Tex. 77025.
c. Semiconductor Art
The prior art of reading charge storage patterns out of multilayer semiconductor sandwich structure lies largely in the area of electronics, especially exotic computer memories.
Charge patterns on certain MOS structures can be "read out" using a photon beam. See, Imaging and Storage With a Uniform MOS Structure, Applied Physics Letters, Vol. 11, Number 11, pp. 359-361. This technology functions by modifying a depletion layer and then charging the layer to saturation. The few micron thick depletion layer is the only active structure.
Electric fields can also be impressed across two separable photoconductive insulating films in pressure contact that are precharged to the same polarity. See, Increasing the Sensitivity of Xerographic Photoconductors, IBM Technical Disclosure Bulletin, Vol. 6, No. 10, 1964, page 60.
IBM has also developed an exotic charge storage beam addressable memory comprising a semiconductor sandwich wherein the semiconductor is totally insulated from both electrodes in the sandwich. See, IBM Technical Disclosure Bulletin, Vol. 9, No. 5, 1966, pp. 555-556. This device allows data readout by shifting charge population to one side or the other of the insulated semiconductor.
Finally, some theoretical work has been done on the behavior of light sensitive capacitors, see, Analysis and Performance of a Light Sensitive Capacitor, Proceedings of the IEEE, April 1965, p. 378.
d. Objects of the Present Invention
It is an object of the present to provide a method and apparatus capable of replacing conventional radiographic films.
Another purpose of the present invention is to provide an X-ray sensing system whose output is an analog or digital video signal that may be selectively displayed on a television monitor, recorded on film, or directly stored or processed in a computer for image enhancement or pattern recognition.
A further purpose of the present invention is to provide a novel method and apparatus for converting a charge distribution on a semi-conducting surface to a modulated electric signal.
Yet a further purpose of the present invention is to provide a novel low noise method and apparatus for reading out a latent image stored as a pattern of electrical charges by selectively discharging said charges in the presence of a reverse biased electric field.
A final purpose of the present invention is to provide an X-ray sensing system that is capable of directly timing X-ray exposure.